1. Most PTO's require 18 and up for judging. The professor program used to allow 15 year olds to be part of that program. But they upped it to 18.
2. Yes. Most tournaments you judge are worth 18 packs of cards. Larger tournaments like states are usually worth a box. It depends on the PTO as to how much you get. Judge support is out of pocket for them.
3. Judges help make ruling decisions during tournaments. They are there to answer questions players may have, and keep track of time.
4. If you are judging a tournament, you aren't able to play in it. Besides giving you the possibility of an unfair advantage, you can't keep getting up during games to answer questions for people.